Why did Lois Lerner's personal email account contain confidential taxpayer information?

At least Richard Windsor's email account did not contain confidential personal taxpayer information. The same cannot be said for the private email accounts of Lois Lerner and Doug Schulman. Congressman Darrell Issa Chairman of the House Oversight and Government Reform Committee said the committee’s investigation produced more than 1,600 pages of emails and documents housed in Lerner’s nonofficial email account related to IRS business, including nearly 30 pages of confidential taxpayer information. Included in the material was a summary of an application for tax-exempt status the IRS instructed Lerner’s legal counsel to redact. More.